<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    .box1 {
      color: rgba(255, 0, 0, .5);
    }

    .box2 {
      /* 新的 color module 的写法 */
      /* 不推荐 */
      color: #ff000088;
    }

    .box3 {
      /* 通过颜色隐藏 */
      color: rgba(0, 0, 0, 0);
    }

    .box4 {
      /* transparent -> rgba(0, 0, 0, 0) */
      background-color: transparent;
    }

    .box5 {
      /* 所有子元素 */
      opacity: 0.5;
    }
  </style>
</head>

<body>
  <div class="box1">我是box1</div>
  <hr>
  <div class="box2">我是box2</div>
  <hr>
  <div class="box3">我是box3
    <img src="../images/gouwujie01.jpg" alt="">
  </div>
  <hr>
  <div class="box4">我是box4</div>
  <hr>
  <div class="box5">我是box5
    <img src="../images/gouwujie01.jpg" alt="">
  </div>
</body>

</html>